It is well known in the art of vehicle design that the fuel consumption of a vehicle associated with its movement is directly related to certain aerodynamic characteristics of the vehicle, such as the aerodynamic drag of the vehicle expressed as the drag coefficient, Cd. As the aerodynamic drag experienced by a vehicle increases, the fuel costs also correspondingly increase due to the greater energy required to overcome the drag. For example, for a vehicle traveling 70 mph on a roadway, approximately 65% of the total fuel consumption of its engine is used to overcome aerodynamic drag. Thus, even a slight reduction in the aerodynamic drag coefficient of the vehicle can result in a significant improvement in fuel economy. This is especially true for bluff body vehicles, such as semi-trailer trucks (“semi's”) and other heavy vehicles having a tall and wide frontal profile. FIG. 1 shows a conventional semi-trailer truck 10 having a tractor-trailer arrangement with a trailer 13 hitched to a tractor 11. The tractor 11 has a drive wheel assembly 12, and the trailer 13 is a semi-trailer type having a trailer body 14 with a rear end 16 supported by a rear wheel assembly 17 and a front end 15 (without a front axle) hitched to the tractor 11 above the tractor's drive wheel assembly 12.
One of the sources of aerodynamic drag on a vehicle is underbody drag caused by airflow separation from a forward-located vehicle wheel assembly (e.g. a front axle) which forms a recirculation zone under the vehicle body directly behind (immediately downstream of) the wheel assembly and reduces the pressure on the rear or base of the wheel assembly. In the case of heavy vehicles such as the semi 10 in FIG. 1, the underbody recirculation zone 18 shown under the trailer body 14 directly behind the tractor's drive wheel assembly 12 is particularly large due to the high ground clearance of the trailer body and the sizeable (tall and wide) cross-sectional area of the tractor's drive wheel assembly. One device known in the art for reducing such trailer underbody drag is an underbody skirt, such as 19 shown in FIGS. 2 and 3, that extends below the trailer body 14 of the semi 10, one on each side of the trailer body. However, as also shown in FIGS. 2 and 3, a potential problem with such underbody skirts is inadequate ground clearance when the semi travels over a raised, protruding, undulating, or otherwise uneven part of the travel surface. This is due to the lower position of the skirt relative to the trailer body, together with the relatively long wheel base, i.e. the distance between the tractor's drive wheel assembly and the trailer rear wheel assembly. In particular, FIG. 2 illustrates the underbody trailer skirt 19 coming in contact with a railroad track at a railroad crossing 2, and FIG. 3 illustrates the underbody trailer skirt 19 coming in contact with a transition surface of a ramp leading into a sunken loading dock 3. In either case, substantial damage may result to the skirt due to inadequate ground clearance.
Thus there is a need for an aerodynamic drag reduction apparatus which reduces underbody drag of a vehicle, especially heavy vehicles such as semi-trailer trucks, caused by a wheel assembly supporting the vehicle body, while also overcoming the ground clearance problem.